Polymers and oligomers having carbamate functional groups have been used in a variety of curable compositions. Carbamate-functional acrylic polymers [as used herein the terms `acrylic polymer` and `acrylic monomer` are defined to include derivatives of both acrylic acid and methacrylic acid] are described, for example, in U.S. Pat. 5,356,669 and WO 94/10211. Carbamate-functional polymers offer many advantages in such compositions, such as resistance to environmental etch, resistance to scratching and marring, and other beneficial physical properties.
Many carbamate-functional polymers described in the art are addition-type polymers. Addition-type polymers are prepared by free radical addition to the double bonds of monomers containing unsaturated sites. Two techniques have been described in the art for preparing carbamate-functional polymers. According to one technique, a monomer is prepared having a carbamate-functional group and polymerized, optionally along with other monomers having groups that are non-reactive with carbamate. Such monomers are well-known in the art and are described, for example in U.S. Pat. Nos. 3,479,328, 3,674,838, 4,126,747, 4,279,833, and 4,340,497, the disclosures of which are incorporated herein by reference.
One method of synthesis of such monomers involves reaction of a hydroxy ester with urea to form the carbamyloxy carboxylate (i.e., carbamate-modified acrylic). Another method of synthesis reacts an a,b-unsaturated acid ester with a hydroxy carbamate ester to form the carbamyloxy carboxylate. Yet another technique involves formation of a hydroxyalkyl carbamate by reacting a primary or secondary amine or diamine with a cyclic carbonate such as ethylene carbonate. The hydroxyl group on the hydroxyalkyl carbamate is then esterified by reaction with acrylic or methacrylic acid to form the monomer. Other methods of preparing carbamate-modified acrylic monomers are described in the art, and can be utilized as well. The acrylic monomer is then polymerized along with other ethylenically-unsaturated monomers, if desired, by techniques well-known in the art.
Another route for preparing a carbamate-functional polymer is to react an already-formed polymer such as an acrylic polymer with another component to form a carbamate-functional group appended to the polymer backbone, as described in U.S. Pat. No. 4,758,632, the disclosure of which is incorporated herein by reference. One technique for preparing polymers useful as component (a) involves thermally decomposing urea (to give off ammonia and HNCO) in the presence of a hydroxy-functional acrylic polymer to form a carbamate-functional acrylic polymer. Another technique involves reacting the hydroxyl group of a hydroxyalkyl carbamate with the isocyanate group of an isocyanate-functional acrylic or vinyl monomer to form the carbamate-functional acrylic. Isocyanate-functional acrylics are known in the art and are described, for example in U.S. Pat. No. 4,301,257, the disclosure of which is incorporated herein by reference. Isocyanate vinyl monomers are well-known in the art and include unsaturated m-tetramethyl xylene isocyanate (sold by American Cyanamid as TMI.RTM.). Yet another technique is to react the cyclic carbonate group on a cyclic carbonate-functional acrylic with ammonia in order to form the carbamate-functional acrylic. Cyclic carbonate-functional acrylic polymers are known in the art and are described, for example, in U.S. Pat. No. 2,979,514, the disclosure of which is incorporated herein by reference. Yet another technique is to transesterify a hydroxy-functional acrylate polymer with a carbamate compound.
Although many of the above-described techniques can be effectively used to prepare carbamate-functional polymers, there are difficulties in applying these techniques. The solubility of carbamate-functional monomers often has poor compatibility with that of many other monomers typically incorporated in addition polymers (e.g., styrene, isobornyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate). This can restrict the level of monomer that can be incorporated in the polymerization mixture and/or the choice of solvent, sometimes to certain polar and/or UV-sensitive solvents that can have adverse effects.
One approach to prepare carbamate-functional polymers without the use of a carbamate-containing monomer has been to transesterify a hydroxy-functional acrylic polymer with a carbamate (e.g., an alkyl carbamate such as methyl carbamate). However, such polymers can exhibit undesired gains in molecular weight. Although an understanding of this molecular weight gain is not needed to characterize the present invention, it is theorized as the result of transesterification of the acrylate ester groups. It would thus be desirable to provide a different method of preparing a carbamate-functional addition polymer than the methods described above.